Excessive start to opera: composer’s last in different form? (8)
I believe the answer is:
overdone
'excessive' is the definition.
(thesaurus)
I don't understand the rest of the clue.
(Other definitions for overdone that I've seen before include "Exaggerated; burnt (meat)" , "Hammed up" , "Cooked too long" , "Cooked too much or carried too far" , "really a bit much!" .)